The present invention relates to a pneumatic booster used for a brake system of a vehicle. Specifically, the present invention relates to a through-rod type pneumatic booster comprising a shell body formed by a front shell and a rear shell which are connected by means of a rod so as to increase rigidity of the shell body.
Conventionally, as indicated in Japanese Patent Application Public Disclosure No. 56-163947, a through-rod type pneumatic booster comprises a front shell and a rear shell, which form a shell body. A power piston is provided in the shell body. The front shell and the rear shell are connected by means of a rod which extends through the power piston. A stud bolt is formed on one end of the rod, and extends through the front shell to the outside. A support member is fitted around the stud bolt. The support member is engaged with the rod and abutted against an inner surface of the front shell. An annular groove is formed in the support member. A seal member is provided in the annular groove so as to seal a space between the front shell and the stud bolt.
FIG. 7 shows a seal structure formed around the stud bolt in the through-rod type pneumatic booster. In FIG. 7, reference numeral 1 denotes a front shell which forms a shell body; 2 a rod connecting the front shell 1 and a rear shell (not shown); 3 a stud bolt formed on one end of the rod 2; 4 a support plate (a support member) fitted around the stud bolt 3; and 5 a seal member held by the support plate 4. The support plate 4 is engaged with an end face (a stepped surface) 2a of the rod 2, with a surface 4a thereof abutting against an inner surface of the front shell 1. The seal member 5 is provided in an annular groove 6 formed at a radially inner portion of the surface 4a of the support plate 4, to thereby seal a space between the front shell 1 and the stud bolt 3. The stud bolt 3 is used for mounting a master cylinder M on the pneumatic booster. A flange F of the master cylinder M and the front shell 1 are fastened between the support plate 4 and a nut N threadably engaged with the stud bolt 3.
After assembly, the pneumatic booster is subjected to a leak inspection prior to shipment. In the above-mentioned seal structure in which the seal member 5 is provided in the support plate 4, even when the support plate 4 is reversely mounted on the rod 2 as shown in FIG. 8, a space between the front shell 1 and the stud bolt 3 is sealed by the seal member 5, and the booster passes the leak inspection. In this case, however, as shown in FIG. 8, a space S is formed between the support plate 4 and the front shell 1, so that the support plate 4 is unable to support the front shell 1. Therefore, the front shell 1 is likely to be deformed due to application of external force, and therefore, a hermetic seal between the front shell 1 and the stud bolt 3 may be impaired.